1. Field of the Invention
The present invention relates to a tractor travelling system transmission mechanism comprising a friction type stepless transmission means interlocked with an engine, a planet reduction gearing and a forward/reverse changeover gearing, wherein in the planet reduction gearing a first component element thereof is interlocked with the engine, a second component element thereof is interlocked with the stepless transmission means and a third component element thereof is interlocked with the travelling wheels.
2. Description of the Prior Art
In order to steplessly change the travelling speed of a tractor without operating clutch means, it may be considered to use a hydraulic stepless transmission mechanism. However, there is a disadvantage that a tractor itself becomes expensive due to costly hydraulic machinery to be used therein.
Therefore, in order to steplessly change the travelling speed by using economical mechanical structures, several methods have been researched and developed by utilizing a friction type stepless transmission means, a planet reduction gearing and a forward/reverse changeover gearing. However, in such transmission mechanisms there has been also a problem in the forward/reverse changeover system as described below.
Among conventional transmission mechanisms of this kind, there is one type in which the forward/reverse changeover is also provided by the friction type stepless transmission mechanism. In such a transmission mechanism a follower shift roller is contacted with a disc rotated at a uniform speed, and the contact point of the shift roller with the disc is radially changed, thereby to provide stepless speed change. Consequently, when intending to provide the forward/reverse changeover by this friction type stepless transmission mechanism, it is required to change this contact point of the shift roller while passing it across the center point of the disc, and subsequently the contact area of the shift roller with the disc becomes very broad. It is therefore inevitable in such a transmission mechanism to use a low speed transmission area of low power transmission efficiency, thereby to easily provoke slips, thus providing no accurate speed change operation.
It may also be considered to arrange a transmission mechanism in which forward/reverse changeover is provided by changing the direction of the revolution of planet gears, which are one component element of the planet reduction gearing, around the sun gear. However, in order to change the direction of the revolution of the planet gears around the sun gear it is necessary to greatly change the number of revolutions of the internal gear of the planet reduction gearing, requiring also a broad transmission area of the friction type stepless transmission mechanism which controls the number of revolutions of this internal gear. Therefore, a defect similar to the abovementioned one would not be overcome.